Chatoyancy or chatoyance, also known as the "cat's eye effect", is a visual effect seen in certain minerals, such as Tiger's Eye. When polished, chatoyant stones exhibit a bright streak of light that seems to move as you turn it.
Chatoyancy appears in minerals that have a fibrous nature, or fibrous inclusions. When the fibers are all aligned in the same direction, they reflect a streak of light that runs perpendicular to the alignment of the fibers.
The cabochon form - a smooth, polished dome-shape - is used to display chatoyancy. Make sure the fibers are aligned parallel with the base of the stone, for the best effect.
Quartz frequently displays this effect, as does beryl, tourmaline, moonstone, and scapolite. Rarely, nephrite jade shows chatoyancy, particularly the material from the Big Sur Coast. Although beautiful, chatoyant jade can prove somewhat fragile and difficult to work with.